In microwave oscillators and the like utilizing microwave cavities in housings it is necessary to provide some temperature compensation for changes in the resonant frequency of the cavity due to temperature changes of the housing or other components. In prior art structures it is common to slideably mount a tuning rod in an opening in the housing by means of an external compensating device which causes movement of the rod in the cavity in response to temperature changes of the compensating device. In many instances, to prevent radiation leakage through the opening in the housing, the structure is partially formed from or encircled by metal. This metal shields the structure to prevent radiation but does not prevent losses of energy through the opening. Further, these devices are generally complicated and expensive to manufacture.